Thick film, multi-layer ceramic interconnected circuit boards, in general, include a substrate of an insulating material, such as alumina, which can withstand high temperatures having on a surface thereof a plurality of thick film conductor layers separated by layers of a dielectric material. The thick film conductor layers are generally formed of a mixture of a glass frit and particles of a conductive material which is fired on the substrate to form a layer of glass having the conductive particles dispersed therethrough. The dielectric layers are generally of a glass fired on the substrate. The dielectric layers have openings or "vias" therethrough which are filled with a conductive material to connect the conductive layers in a desired circuit arrangement. The conductive layers are in the form of a circuit pattern, and can include areas forming resistors and capacitors. On the outermost dielectric layer are contact pads of a conductive material which are connected to the underlying conductive layers by the conductive material in the vias. Electrical components, such as integrated circuits, transistors, diodes, capacitors, resistors and the like, are mounted on and soldered to the contact pads. The contact pads are also generally made of a thick film material which is not only electrically conductive, but is also easily soldered to.
There are a number of concerns with regard to this type of interconnected circuit board. One concern is that the contact pads be strongly bonded to the board. Since the thick film conductive material used for the contact pads does not adhere well to the glass generally used for the dielectric layer, it has been the practice to have the contact pads completely cover the via conductive material which extends completely through one or more of the layers of the circuit board and perhaps even to the substrate. Thus, the area of the via conductive material is generally the same as the area of the contact pads. In order to bond the electrical components to the contact pads, the pads must be relatively large. Therefore, the area of the via conductive material is relatively large so as to take up considerable room on the board. This undesirably makes more difficult the layout of the circuit conductors, and increases the cost of the board because large amounts of the conductive material, which is very expensive, are required. Another problem is the inability to remove and replace components on the contact pads so as to repair or otherwise change the circuit on the board. I have discovered that when a component is unsoldered and replaced on a thick film contact pad about two or three times, the thick film conductive material of the contact pad becomes unbonded from the underlying dielectric layers rendering the circuit board unusable.